1. Field of the Invention
This invention relates to an image forming apparatus having an intermediate transfer member and an image forming method applied to the image forming apparatus.
2. Related Background Art
Image-forming apparatus making use of an intermediate transfer belt are effective as full-color image forming apparatus or multi-color image forming apparatus in which a plurality of component-color images corresponding to full-color image information or multi-color image information are sequentially superimposingly transferred to a transfer medium to output an image-formed material on which a full-color image or multi-color image has synthetically been reproduced, or as image forming apparatus made to have the function of full-color image information or the function of multi-color image information.
Compared with image forming apparatus in which toner images are transferred from a first image bearing member (photosensitive member) to a second image bearing member (transfer medium) stuck or attracted onto a transfer belt (see, Japanese Patent Application Laid-open No. S63-301960, etc.), the image forming apparatus making use of an intermediate transfer belt have an advantage that a great variety of second image-bearing members (transfer mediums) can be selected without regard to their width and length, including thin paper (40 g/m2 paper) and up to thick paper (200 g/m2 paper) such as envelopes, post cards and labels. This is because it is unnecessary to make any processing or control for the second image-bearing member transfer medium (e.g., to hold the transfer medium with a gripper, hold it by attraction, and make it to have a curvature).
In addition, taking the form of an intermediate transfer belt enables effective utilization of space to make the apparatus main body compact and achieve cost reduction, because the freedom in disposing it inside the image-forming apparatus can be greater than a case in which a rigid-body cylinder such as an intermediate transfer drum is used.
Further proposed is a system in which an intermediate transfer belt having an elasticity is used. This enables securement of a sufficient transfer region, what is called a transfer nip, at a primary transfer zone between the first image bearing member such as a photosensitive member and the intermediate transfer belt and at a secondary transfer zone where the toner images are transferred to the second image bearing member.
In particular, in the case of full-color images on which toners are laid in a large quantity over the whole image areas, the above system can solve the problems that partial faulty transfer tends to occur when conventional intermediate transfer belts having no elasticity are used, color tones may differ, and white-hollowed images, or “blank areas caused by poor transfer (or hollow characters)” may occur in which center areas of lines of character images are not transferred and only edge areas thereof are transferred.
For example, Japanese Patent Applications Laid-open No. H11-231683 and No. H11-024429 disclose intermediate transfer belts having an elasticity. However, although transfer performance and hollow characters have certainly been improved or remedied in both the primary transfer and the secondary transfer, coarse images due to transfer have not been remedied.
Now, in the cleaning of toner carrying members, a blade method or a fur brush method is commonly used. The blade method is a method in which a cleaning blade such as a rubber blade is pressed against the toner carrying member to scrape toners off mechanically. As for the fur brush method, it is a method in which a cylindrical substrate with a fur brush stuck to its surface is brought into contact with the object member while being rotated and at the same time a potential difference is provided between the fur brush and the object member to be cleaned so as to attract the toner towards the fur brush side, to remove the toner from the object member mechanically or electrostatically.
In the cleaning of an elastic intermediate transfer belt, the use of the blade method causes the cleaning blade to have a large contact load against the elastic intermediate transfer belt, and the cleaning blade comes into contact with the elastic intermediate transfer belt at so strong a frictional force that the former may come to eat into the latter. This tends to, e.g., cause an increase in torque at the time of start, make the blade caught in, cause streaks due to friction scratches and make the belt move to one side because of non-uniform friction, any of which can be one of the causes that make the belt have a short lifetime. Without limitation to such an elastic intermediate transfer belt, intermediate transfer belts are component parts which require a relatively high cost. Hence, a shorter lifetime of intermediate transfer belts results in shorter intervals at which the intermediate transfer belts are to be replaced, to exercise a great influence on the cost of image formation. The intermediate transfer belts are component parts which are replaced at regular intervals, and it follows that the cost of the intermediate transfer belts is added to the cost of image formation. In recent years, it is highly required to reduce the cost of image formation also in regard to color image formation, and hence it is required to make the intermediate transfer belt have a longer lifetime. Accordingly, in order to clean the elastic intermediate transfer belt, a cleaning method that may give a small contact load is preferred. In comparison of the above two methods, the fur brush method is more suited than the blade method.
Japanese Patent Applications Laid-open No. H10-254251, No. H8-292623 and No. 2002-229344 disclose methods of belt cleaning performed by the fur brush method. All of these can enjoy a small contact load, and no longer make the elastic intermediate transfer belt have a short lifetime. However, these have not been satisfactory for the cleaning of elastic intermediate transfer belts in high-speed electrophotographic equipment.